Conventionally, a slot type spindle motor, in which a yoke (stator yoke) provided with salient poles is used in a stator and which has excellent magnetic efficiency, is generally employed in magnetic disk apparatuses. However, with increasing magnetic recording density, the slot type spindle motor tends to suffer from cogging which is generated by the presence of the salient poles and which causes an undesirable vibration. To prevent such a cogging, a slotless spindle motor is disclosed in which a sheet-like coil is fixedly attached at the outer circumferential surface of a stator yoke which is free from salient poles (refer to, for example, Japanese Patent Application Laid-Open No. 2002-27720).
FIG. 13 is a perspective view of a slotless spindle motor 71 disclosed in Japanese Patent Application Laid-Open No. 2002-27720. The spindle motor 71 is a so-called radial gap type motor and includes a back yoke (stator yoke) 73 which is made of a magnetic material and formed into a circular cylinder, a coil sheet (sheet-like coil) 74 which is tightly fixed to the outer cylindrical surface (outer circumferential surface) of the back yoke 73, a magnet (rotor magnet) 75 which is formed into a circular cylinder and magnetized with a plurality of magnetic poles arranged in the circumferential direction, and a yoke (hub) 76 which retains the magnet 75 and at the same time functions as a turntable. The back yoke 73 and the magnet 75 are disposed concentrically to a rotary shaft 77 and a bearing 78, wherein a predetermined space (gap) is maintained between the back yoke 73 and the magnet 75.
The coil sheet 74 is structured such that a copper foil is formed by an etching or transferring method on a thin base film made of a soft and flexible material, for example, polyester or polyimide whereby a plurality of planar coils are built on the thin base film, and that a thin cover film is formed over the coils to prevent peeling-off and short circuit.
Since the coil sheet 74 is disposed in the magnetic field formed by the magnet 75 and the back yoke 73, when current is caused to flow in the planar coils formed in the coil sheet 74, an electromagnetic force is generated to act on each of the planar coils by Fleming's left-hand rule in the rotation direction. Due to the reaction of the electromagnetic force acting on each planar coil, the magnet 75 and the yoke 76 are rotated, wherein the magnetic field is uniformly distributed because the back yoke 73 has an even outer circumferential surface (no salient poles). Consequently, the motor is adapted to rotate smoothly without cogging.
In the slotless spindle motor 71 with the coil sheet 74, however, it is hard to enhance the magnetic density in the space between the coil sheet 74 and the magnet 75 compared with in the slot type spindle motor. Therefore it is hard to increase torque in the slotless spindle motor 71. To overcome the above problem with the slotless spindle motor 71, the number of coil turns must be increased to achieve an increased torque. However, in a typical sheet coil which is made such that a single-layered coil is formed on a flexible substrate by printing or etching, it is hard to increase the number of coil turns, which means it is difficult to achieve a high-density conductor circuit with coil having a high wiring occupation ratio.
With respect to the foregoing, for example, an FP coil (fine pattern coil by Asahi Kasei Electronics Co., Ltd.) is known, which is a sheet coil adapted to realize a high aspect-ratio electric conductor (thick film conductor) as well as a short distance between adjacent coil turns of the conductor and therefore achieves a high-density multi-layered thick film conductor circuit. Further, a motor to use the FP coil as planar coils is proposed for achieving a high torque (refer to, for example, Japanese Patent Application Laid-Open No. H8-222425).
FIGS. 14A and 14B show a motor 91 disclosed in Japanese Patent Application Laid-Open No. H8-222425, wherein FIG. 14A is an axial cross sectional view of the motor 91 and FIG. 14B is a plan view of a planar coil. The motor 91 is a so-called axial gap type motor and includes a yoke base plate 92 made of a metal plate and the like, a thick film fine pattern coil 93 fixed on the yoke base plate 92, and a magnet 97 rotatably disposed to oppose the thick film fine pattern coil 93.
The thick film fine pattern coil 93 includes a plurality of triangular coil conductors 94 disposed equiangularly on the same plane with a predetermined interval, a plurality of terminals 95 and a plurality of wiring lines 96 connecting between the coil conductors 94 and the terminals 95. The thick film conductors have a cross section of a substantially rectangular shape with a height of 150-320 μm, wherein a minimum cyclic pitch between adjacent conductor turns ranges from 100 to 200 μm.
However, it turns out that when such a thick film fine pattern coil is used in a radial gap type spindle motor which is expected to generate a larger torque than the above described axial gap type spindle motor, there occurs a problem that when the strip-shaped thick film fine pattern coil, which includes a plurality of rectangular individual coils arrayed in a straight line, is bound around the outer cylindrical circumference of a back yoke, the thick film fine pattern coil does not make a tight contact with the entire circumferential surface of the back yoke thus resulting in unintentionally and undesirably generating gaps partly therebetween. This is because the thick film fine pattern coil, when rolled up, undergoes an inhomogeneous deformation, for example, may be bent between every adjacent individual coils thus failing to be rolled up in a good shape entirely with a uniform curvature. If the whole surface of the thick film fine pattern coil is not brought into a tight contact with the back yoke, it is hard to narrow an air gap between the back yoke and the magnet, and therefore a magnetic flux density enhancement which is supposed to result from narrowing of the air gap is inhibited. Also, torque ripple is generated, and consequently it is very hard to achieve a stabilized rotation performance.
Also, depending on the degree of deformation, the thick film fine pattern coil is possibly cracked at portions each located between two adjacent individual coils, in which case the thick film conductor may be broken or rusted. Further, gas or dust may leak from the cracked portions.
The problem that the thick film fine pattern coil, when rolled up, is deformed and eventually cracked at the portions between two adjacent individual coils as described above is attributed to the difference in stiffness between the metal material (thick film conductor), such as copper, for forming the individual coils and the resin material (insulation layer between conductors) present between the adjacent individual coils.
On the other hand, the thick film fine pattern coil entirely has relatively large stiffness, so that when the thick film fine pattern coil is rolled up to sit tightly on the outer cylindrical surface of the back yoke (especially if its diameter is small), both ends of the thick film fine pattern coil are possibly caused to spring back. Consequently, the thick film fine pattern coil may be peeled off from the back yoke.